Method Of Manufacturing Of A Sintered Metal Fiber Medium

ABSTRACT

The invention relates to a method for manufacturing a sintered metal fiber medium, comprising the steps of: providing metal fibers; making a slurry comprising the metal fibers and a binding agent by mixing the metal fibers and the binding agent, possibly with a solvent; casting a layer of the slurry on a support using an applicator; solidifying this slurry, providing a foil comprising the metal fibers and the binding agent; debinding the binding agent in the foil and sintering the metal fibers.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing of a sinteredmetal fiber medium.

BACKGROUND OF THE INVENTION

Sintered metal fiber media are well known in the art for numerousapplications, such as e.g. liquid or gas filtration.

A first method for providing sintered metal fiber medium is to provide ametal fiber web by air lay down, and sintering this air laid web inappropriate furnaces.

A disadvantage of this air lay down web, is the fact that the web isusually relatively inhomogeneous, especially when relatively thinsintered metal fiber medium are to be provided. This because the airlaid webs can hardly be provided sufficiently homogeneous, andtherefore, to have a sintered metal fiber medium with homogenousproperties over its surface, usually several air laid webs are stacked(so-called doubled).

An other method to provide a web, prior to sintering operation, is touse the so-called wet lay down method or paper making method, asdescribed in WO98/43756, EP933984A, JP11-131105, JP61-225400 andJP61-223105. The metal fibers are brought in a slurry, which slurry ispoured on a screen. The water is sucked from the slurry through thescreen. The remaining dewatered slurry is then sintered. A binding agentmay be used to temporarily bind the metal fibers to each other and so tomake the dewatered slurry transportable. This dewatered slurry is thensintered, possibly first debinding the binding agent.

A disadvantage of the wet webbing is that in case that thin andrelatively short fibers are used, some of the shorter fibers are suckedthrough the screen, together with the water being removed from theslurry. In case of thin webs made prior to sintering, the dewateringstep may suck small or larger holes in the web where few or no fibersare retained for sintering. Also, an imprint of the supporting net, usedto support the wet slurry during dewatering, is obtained. The netpattern is noticed on the dewatered web as repetitive thinner spots.

As a result, the dewatered slurry and thus the sintered metal fibermedium, may have inhomogeneous zones where less fibers are present, evenwhen several layers of the freshly dewatered webs are stacked one to theother prior to sintering.

Especially in case fibers with small equivalent diameter, e.g. 2 μm to 6μm, are used, the phenomena of sucking fibers with the water duringdewatering is noticed. This because usually the amount of fibers withsmaller lengths is larger, the finer the fibers are. As a result, morefibers with a short length are sucked with the water during dewateringin case of fibers with small equivalent diameter.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method formanufacturing sintered metal fiber media which overcomes the drawbacksof prior art. It is an object of the present invention to provide amethod of manufacturing a sintered metal fiber medium with homogeneousproperties over its surface. It is also an object of the presentinvention to provide a method of manufacturing a sintered metal fibermedium with homogeneous properties over its surface comprisingrelatively short and/or fine metal fibers. It is further an object ofthe present invention to provide a method of manufacturing a sinteredmetal fiber medium with homogeneous properties over its surface, whichmedium has a relatively small thickness.

A method for manufacturing a sintered metal fiber medium as subject ofthe invention comprises the steps as described in claim 1.

Preferably, the slurry used for casting using an applicator, orso-called tape casting, comprises an amount of metal fibers in the rangeof 2% weight to 40% weight of the slurry, more preferred between 5%weight and 15% weight of the slurry. Apparently, such concentrationcombined with the tape casting action to provide substantially flatlayers of slurry, causes metal fibers to be distributed morehomogeneously, so providing sintered metal fiber medium having morehomogeneous properties over its surface and in depth of the medium.

Too much metal fibers in the slurry may cause conglomeration of thefibers, causing on its turn inhomogeneous metal fiber distributionthroughout the sintered metal fiber medium.

Too little metal fibers in the slurry may cause problems duringdebinding, where too much debinding causes disturbing of the sinteringof the metal fibers. Further, such production of sintered metal fibermedium becomes uneconomic, as too much energy is to consumed fordebinding the binding agent, and a large volume of binding material isto be removed. In each cast layer, the metal fiber distribution over thesurface may become irregular.

In a further preferred method, the slurry comprises a solvent fordissolving the binding agent, and during solidification of the slurry,all solvent is removed by evaporation. This has a further advantageouseffect on the metal fiber distribution homogeneity over the surface andin depth of the sintered metal fiber medium which results from thefurther process.

Considering now the method to provide a sintered metal fiber medium assubject of the invention in more detail.

In a first step, metal fibers are to be provided. Any type of metal ormetal alloy may be used to provide the metal fibers.

The metal fibers are for example made of steel such as stainless steel.Preferred stainless steel alloys are AISI 300 or AISI 400-serie alloys,such as AISI 316L or AISI 347, or alloys comprising Fe, Al and Cr,stainless steel comprising Chromium, Aluminum and/or Nickel and 0.05 to0.3% by weight of Yttrium, Cerium, Lanthanum, Hafnium or Titanium, suchas e.g. DIN 1.4767 alloys or Fecralloy®, are used. Also Cupper orCopper-alloys, or Titanium or Titanium alloys may be used.

The metal fibers can also be made of Nickel or a Nickel alloy. Metalfibers may be made by any presently known metal fiber production method,e.g. by bundle drawing operation, by coil shaving operation as describedin JP3083144, by wire shaving operations (such as steel wool) or by amethod providing metal fibers from a bath of molten metal alloy.

In order to provide the metal fibers with their average length, themetal fibers may be cut using the method as described in WO02/057035, orby using the method to provide metal fiber grains such as described inU.S. Pat. No. 4,664,971.

The metal fibers used to provide the sintered metal fiber medium arecharacterized in having an equivalent diameter D and an average fiberlength L.

With equivalent diameter of a metal fiber is meant the diameter of animaginary circle having the same surface as the surface of a radialcross section of the fiber.

Preferably the equivalent diameter D of the metal fibers is less than100 μm such as less than 65 μm, more preferably less than 36 μm such as35 μm, 22 μm or 17 μ. Possibly the equivalent diameter of the metalfibers is less than 15 μm, such as 14 μm, 12 μm or 11 μm, or even morepreferred less than 9 μm such as e.g. 8 μm. Most preferably theequivalent diameter D of the metal fibers is less than 7μm or less than6 μm, e.g. less than 5μm, such as 1 μm, 1.5 μm, 2 μm, 3μm, 3.5 μm, or 4μm.

The metal fibers all have an individual fiber length. As somedistribution on these fiber lengths may occur, due to the method ofmanufacturing the metal fibers, the metal fibers, used to provide asintered metal fiber medium as subject of the invention, have an averagefiber length L. This length is determined by measuring a significantnumber of fibers, according to appropriate statistical standards. Theaverage fiber length of the metal fibers is smaller than 10 mm, e.g.smaller than 6 mm, preferably smaller than 1 mm, such as smaller than0.8 mm or even smaller than 0.6 mm such as smaller than 0.2 mm. Asaccording to the present invention, substantially all fibers used duringthe method of manufacturing the sintered metal fiber medium will occurin the sintered metal fiber medium, the average fiber length L can bemeasured in a similar way on the sintered metal fiber medium.

The metal fibers in the sintered metal fiber medium thus may have aratio of average fiber length over diameter (UD) being preferably lessthan 110, more preferred less than 100, but usually more than 30. An UDof about 30 to 70 is preferred for metal fibers with equivalent diameterin the range up to 6 μm, in case the metal fibers are obtained by theprocess as described in WO02/057035, hereby incorporated by reference.

In the second step of the method as subject of the invention, a slurryis to be provided. Although not to be understood as limiting, preferablythe slurry, comprising metal fiber, a solvent and a binding agent, has ametal fiber concentration in the range of 2% weight to 40% weight of theslurry. Preferably 5% weight to 15% weight of the slurry is provided bymetal fibers. It was found that the smaller the equivalent diameter ofthe metal fibers, the lower the concentration of metal fibers is kept.Alternatively, the slurry comprises a polymer binding agent and metalfibers, which polymer binding agent is heated to reduce its viscosity.

A binding agent for the purpose of the invention is to be understood asa product for thickening the slurry. Preferably a water soluble bindingagent is used, e.g. polyvinyl alcohols, methyl cellulose ethers,hydroxypropylmethylcellulose, polyethers from ethyleneoxide, acrylicacid polymers or acrylic copolymers. The binding agent is added to thesolvent, in a concentration of preferably between 0.5% weight and 30%weight of the slurry. Most preferred, a binding agent is chosen whichrequires a concentration of less than 20% weight or even less than 15%weight or even less than 10% weight of the slurry, in order to providethe required viscosity. A viscosity range between 1000 cPs and 20000 cPsis preferably used for the slurry. The components of the slurry areblended using appropriate mixing equipment. In case foaming of theslurry occurs, small amounts of a defoaming component is added.

In a third step, the slurry is tape cast using an applicator, such as adoctor blade, on a preferably substantially flat surface. The clearanceof the applicator is kept relatively small, this is preferably between0.2 mm and 6 mm, more preferred between 0.2 mm and 3 mm. The speed ofmovement of the applicator is chosen according to the viscosity of theslurry and the composition of the slurry.

The clearance and thus the thickness of the layer of the slurry ischosen in function of the amount of metal fibers in the slurry, therequired weight per surface unit of the sintered metal fiber medium, andthe required density of the sintered metal fiber medium.

In a next step, the cast slurry is solidified, forming a foil whichcomprises the binding agent and the metal fibers. This is preferablydone by evaporating the solvent. A solvent may be used which evaporateseasily at ambient temperature. Alternatively, the evaporation may beexecuted as a drying step in case water was used as solvent. The dryingor evaporating may be executed or assisted by air-drying or may beforced by heating the cast slurry, e.g. by forcing heated air over thesurface of the cast slurry, or by radiating, e.g. microwave- orIR-radiating. It is understood that only the solvent, e.g. water isremoved, which solvent was not chemically bound to the binding agent. Itis understood that, in case solvent is evaporated, the thickness of thecast slurry is reduced up to some extent, as the volume of the castslurry is reduced to provide the volume of the foil. Alternatively, thebinding agent is solidified by cooling the cast slurry, in case thebinding agent was heated to reduce its viscosity.

In the preferred situation, where all solvent is removed by evaporationor where the binding agent is solidified by cooling, no fibers are lostdue to the mechanically removal of the solvent. This has a furtheradvantageous effect on the metal fiber distribution homogeneity over thesurface and in depth of the sintered metal fiber medium which resultsfrom the further process. As no fibers are removed, in this way the L/Dratio of the fibers in the sintered metal fiber medium is identical tothe UD of the metal fibers used to make the slurry.

Possibly the foil, which can be handled as the binding agentinterconnects the metal fibers sufficiently, may be subjected to apressing action to further reduce the thickness of the foil.

In a final step, the foil comprising the metal fibers and the bindingagent is subjected to thermal treatment, for debinding of the bindingagent, and consecutively to sinter the metal fibers to each other. Suchdebinding and sintering may be done in one thermal operation, or may beexecuted as two consecutive operations, not necessarily being doneimmediately one after the other.

After sintering, the sintered metal fiber medium may further besubjected to a compression, e.g. rolling or calendaring, in order tofurther reduce the thickness of the sintered metal fiber medium, or tosmoothen the surface of the sintered metal fiber medium.

Possibly, several layers of foil may be stacked to form a layeredmedium. The different foils are not to comprise identical metal fibers,nor should they be of an identical metal fiber content per surface unitor volume. The different foils may differ from each other in metalfibers, metal fiber content, thickness, weight and other properties.Possibly, other porous metal structures may be stacked to one or morefoils. As an example, a metal wire mesh, an expanded metal sheet or oneor more layers of air laid web, wet laid web or a layer of metal powdermay be added to the foils comprising metal fibers and a binding agent.

Possibly, a metal foil or a metal plate is added to the stack.

Alternatively, such porous metal structure or metal foil or plate may beadded to the sintered metal fiber medium as subject of the invention,e.g. by sintering such porous metal structure or metal foil or plate tothe sintered metal fiber medium as subject of the invention in a secondsintering operation.

Surprisingly it was found that a metal fiber medium obtained by using amethod as subject of the invention, has an improved homogeneity of itsphysical properties such as air permeability, filtration efficiency,pore size, bubble point pressure and pore distribution.

The thickness of the sintered metal fiber medium may vary over a largerange, but relatively thin sintered metal fiber medium may be obtained,e.g. sintered metal fiber medium with thickness less than or equal to0.2 mm or even less than or equal to 0.1 mm. Even more surprising, itwas found that sintered metal fiber media having such thickness lessthan 0.2 mm or less than 0.1 mm, a bubble point pressure of more than10000 Pa may be obtained. It was also notices that a high filtrationefficiency may be obtained when such sintered metal fiber media having athickness less than 0.2 mm or less than 0.1 mm are used as a liquidfilter.

The bubble point pressure is measured using according to the ISO 4003testing method.

The weight of the sintered metal fiber medium as subject of theinvention is preferably less than 500 g/m², more preferred less than 400g/m² or even less than 300 g/m², such as less than 100 g/m² such asabout 3Og/m².

The porosity of the sintered metal fiber medium may vary over a largerange, but it was found that such sintered metal fiber medium may have55% to 80%, such as in the range of 55% to 70%. Without applying arolling or pressing operation to the foil or sintered metal fibermedium, porosities of 80 to 99% may be obtained. Lower porosities may beobtained by applying a rolling or pressing operation to either the foiland/or the sintered metal fiber medium.

The term “porosity” P is to be understood as

P=100*(1-d)

wherein

d=(weight of 1 m³ sintered metal fiber medium)/(SF)

wherein

SF=specific weight per m3 of alloy out of which the metal fibers of thesintered metal fiber medium are provided.

As the sintered metal fiber medium may be used for surface filtration insolid-liquid filtration.

A sintered metal fiber medium as subject of the invention may have amean flow pore size of less than 2 times the equivalent diameter D.

Preferably it was found that the sintered metal fiber medium has a meanflow pore size of less than 1.5 times said equivalent diameter D. Morepreferred, the mean flow pore size of the sintered metal fiber media isequal or less than the equivalent diameter D of the metal fibers of thesintered metal fiber medium, increased by one μm.

The mean flow pore size is measured using a “Coulter Porometer II”testing equipment, which performs measurements of the mean flow poresize according to ASTM F-316-80.

In the preferred case, when the mean flow pore size of less than 2 timesthe equivalent diameter D and when the metal fibers in the sinteredmetal fiber medium have a ratio of average fiber length over diameter(UD) which is preferably less than 110, more preferred less than 100,but usually more than 30, surprisingly it was found that such sinteredmetal fiber media can be cleaned repetitively, e.g. by back flush, backflush or back pulse, with high efficiency and apparently with arestricted or even no particles retained after cleaning. Especially whenthe method is used in which all the solvent is removed by evaporation.

An L/D of about 30 to 70 is preferred for metal fibers with equivalentdiameter in the range up to 6 μm, in case the metal fibers are obtainedby the process as described in WO02/057035, hereby incorporated byreference.

Advantageously the outer surface of the sintered metal fiber medium, tobe used as inflow side of the medium when used for solid-liquid surfacefiltration, has a substantially flat surface. With substantially flat ismeant that the Ra value measured over a statistically relevant lengthless than three times the equivalent diameter D of the metal fibers ofthe sintered metal fiber medium. More preferred, Ra value of the firstouter surface of the sintered metal fiber medium is less then theequivalent diameter D, for example less than 0.5 times the equivalentdiameter D.

Ra value is defined as the arithmetic mean deviation of the surfaceheight from the mean line through the measured profile from the measuredlength. The mean line is defined so that equal areas of the profile lieabove and below the line.

A sintered metal fiber medium obtained by the method as subject of theinvention may advantageously be used as filter medium, for filtration ofparticulates from fluids, either gas or liquid, e.g. by surfacefiltration. As an example, the sintered metal fiber medium may be usedfor soot filtration, or for filtration of beverages, such as beer, wine,or for filtration of oils or coolants. The sintered metal fiber mediummay also be used in fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIGS. 1, 2, 3, 4 and 5 show schematically the steps of methods assubject of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

An embodiment of the present invention is described hereinafter.

In a method as shown in FIG. 1, in the first step 110 of the method assubject of the invention, metal fibers 111 are provided.

In a next step 120, a slurry 121 was made from metal fibers, a bindingagent and a solvent preferably water.

This slurry was blend, using a blending means 122, for several minutesin order to form a substantially stable slurry.

In step 130, the slurry 121 was provided to an applicator 131, beingdoctor blade and tape cast on a substantially flat and water repellantsurface 132. A cast slurry 1 33 was provided.

In the next step 140, the cast slurry 133 was dried and transformed intoa foil 141, as an example in ambient temperature.

In a next step 150, a thermal treatment was executed in two steps.During the first part, in order to debind the binding agent, the foil141 was subjected to a thermal treatment under ambient atmosphere.Consecutively, the debound material was sintered using a sinteringprocess.

As shown in FIG. 2, in an additional step 210, several foils 141 may bestacked to each other prior to debinding the binding agent.

As shown in FIG. 3, an additional step of compressing, e.g. rolling thesintered metal fiber medium 311 in step 310 may be executed. All othersteps are identical to the steps as described and shown in FIG. 1 andFIG. 2.

As shown in FIG. 4, in a next step after sintering of the metal fibersin the thermal treatment step 150, and possibly after compression step310, a porous metal structure 411, e.g. a mash, a metal foil or a metalplate may be added to the sintered metal fiber medium 311 and sinteredin a second sintering operation to the sintered metal fiber medium.

As shown in FIG. 5, an additional step of compressing, e.g. rolling thefoil prior to debinding in step 510 may be executed. All other steps areidentical to the steps as described and shown in FIG. 1.

It is understood that this compression step 510 may as well be combinedwith all other steps as shown in FIGS. 1, 2, 3 and 4.

Possibly, the sintered metal fiber medium was layered with a metal wiremesh and sintered a second time to attach the mesh to the sintered metalfiber medium.

As an example, a sintered metal fiber medium was provided using themethod as shown in FIG. 4.

In a first step, metal fibers with equivalent diameter of 2 μm, made bymeans of bundle drawing processes, are provided. The endless metalfibers are cut into metal fibers having an average length of 109 μm,using the method of WO02/057035. The metal fibers were provided out ofAISI 316L alloy.

Hereafter, a slurry was made using following composition:

9.09% weight of the slurry being metal fibers, 1.36% weight of theslurry being methyl cellulose ether (being binding agent) 89.55% weightof the slurry being water (being the solvent).

The slurry was tape cast using a doctor blade having a clearance of 1.5mm.

Such cast slurry was solidified by drying to the air for about 24 h.Alternatively, IR-radiation may be used to heat the cast slurry andassist the drying operation. A foil was obtained comprising the bindingagent with chemically bound water and metal fibers. A non-sintered metalfiber medium was obtained having a thickness of 251 μm and having aweight of 127 g/m². The non-sintered metal fiber medium comprised 13%weight of binding agent, and 87% weight of metal fibers.

Several foils were stacked to provide a layered foil of about 400 g/m².

This stack of foils is subjected for about 30 minutes to a temperatureof 400° C. under ambient atmosphere for debinding the binding agent.Consecutively, the debound material was sintered at 1100 C for about 30minutes under H2. The metal fibers of all layers of foil are sintered toeach other.

The sintered metal fiber medium obtained was rolled to a porosity of65%, having a weight of about 369 g/m². The sintered metal fiber mediumhas a bubble point pressure of 9470 Pa and a mean flow pore size of 2.9μm. An Ra of 0.99 μm was obtained.

A metal wire mesh is added to the sintered metal fiber product, andagain subjected to a sintering operation under high vacuum atmosphere atabout 1050° C. for 60 minutes. Alternatively a metal foil or plate issintered to the sintered metal fiber medium. The mesh may as well beadded to the stack of foils made prior to the first sintering operation.

Using similar steps, a sintered metal fiber product may be obtained,when using metal fibers of 1.5 μm diameter, having a substantiallysimilar UD. The obtained sintered metal fiber medium have a weight ofabout 333 g/m² and a porosity of 65%. The sintered metal fiber mediumhas a bubble point pressure of 13609 Pa and a mean flow pore size of 2.4μm.

1. A method for manufacturing a sintered metal fiber medium, comprisingthe steps of: providing metal fibers; making a slurry comprising saidmetal fibers and a binding agent by mixing said metal fibers and saidbinding agent.; casting a layer of said slurry on a support using anapplicator; solidifying said slurry, providing a foil comprising saidmetal fibers and said binding agent; debinding said binding agent insaid foil and sintering said metal fibers.
 2. A method as claimed inclaim 1, wherein the concentration of metal fibers in said slurry is inthe range of 2% weight to 40% weight of said slurry.
 3. A method asclaimed in claim 1, wherein said slurry comprising a solvent dissolvingsaid binding agent.
 4. A method as claimed in claim 3, wherein saidsolidifying of said slurry is done by evaporation of all of said solventfrom said slurry.
 5. A method as claimed in claim 3, wherein saidsolvent is water.
 6. A method as claimed in claim 1, wherein said slurryis provided by heating said binding agent.
 7. A method as claimed inclaim 1, wherein said method comprises an additional step of reducingthe thickness of said foil by a pressing operation.
 8. A method asclaimed in claim 1, wherein said method comprises an additional step ofreducing the thickness of said sintered metal fiber medium.
 9. A methodas claimed in claim 1, wherein said method comprises an additional stepof stacking several foils to each other prior to said debinding of saidbinding agent.
 10. A method as claimed in claim 1, wherein said methodcomprises an additional step of adding a porous metal structure, a metalfoil or metal plate to said foil prior to debinding said binding agent.11. A method as claimed in claim 1, wherein said method comprises anadditional step of sintering a porous metal structure, a metal foil ormetal plate to said sintered metal fiber medium.
 12. A method as claimedin claim 1, wherein the thickness of said sintered metal fiber mediumless than or equal to 0.2 mm.
 13. A method as claimed in claim 1,wherein the porosity of said sintered metal fiber medium is in the rangeof 40% to 99%.
 14. A method as claimed in claim 1, wherein the bubblepoint pressure of said sintered metal fiber medium is more than 10000Pa.
 15. A method as claimed in claim 1, wherein the mean flow pore sizeof said sintered metal fiber medium is less than 1.5 times saidequivalent fiber diameter D of said metal fibers.
 16. A method asclaimed in claim 1, wherein the metal fibers have an UD of less than110, said L being the average fiber length.